Semiconducting materials are used in a variety of applications, and may be incorporated, for example, into electronic devices such as photovoltaic devices. Photovoltaic devices convert light radiation into electrical energy through the photovoltaic effect.
A photovoltaic device may comprise silicon, for example, as a semiconducting material. For silicon-based devices, silicon may be formed into a variety of shapes using a variety of techniques. Examples include silicon formed as an ingot, sheet, or ribbon. The silicon may be supported or unsupported by an underlying substrate. However, conventional methods of making supported and unsupported articles of silicon have a number of shortcomings.
Methods of making unsupported articles of semiconducting material, such as, for example, silicon sheets, may be slow or wasteful of the semiconducting material feedstock. Unsupported single crystalline semiconducting materials can be produced, for example, using the Czochralski process. However, such bulk methods may disadvantageously result in significant kerf loss when the material is cut into thin sheets or wafers. Additional methods by which unsupported polycrystalline semiconducting materials can be produced include electromagnetic casting and ribbon growth techniques. However, these techniques tend to be slow and expensive. Polycrystalline silicon ribbon produced using silicon ribbon growth technologies is typically formed at a rate of only about 1-2 cm/min.
Supported semiconducting material sheets may be produced less expensively, but the semiconducting material sheet may be limited by the substrate on which it is formed, and the substrate may have to meet various process and application requirements, which may be conflicting.
Methods for producing unsupported polycrystalline semiconducting materials are disclosed in commonly-owned U.S. Provisional Patent Application No. 61/067,679, filed Feb. 29, 2008, titled “Method of Making an Unsupported Article of a Pure or Doped Semiconducting Element or Alloy,” and PCT Publication No. WO09/108,358, published Sep. 3, 2009, titled “Methods of Making an Unsupported Article of Pure of Doped Semiconducting Element or Alloy,” the disclosures of which are hereby incorporated by reference.
The properties of semiconducting materials may depend on a variety of factors, including crystal grain structure, the concentration and type of intrinsic defects, and the presence and distribution of dopants or other impurities. Within a semiconducting material, the crystal grain size, grain size distribution, and grain orientation, for example, can impact the performance of resulting devices. By way of example, the electrical conductivity and thus the overall efficiency of a semiconductor-based device, such as a photovoltaic cell, will generally improve with larger and more uniform grains.
It is known that an increase in production throughput may lead to a decrease in crystal grain quality and, therefore, a decrease in the efficiency of resulting semiconductor-based devices. One solution is to decouple the process of making an article of semiconducting material (e.g., silicon sheet) from the process of improving grain structure and/or otherwise minimizing defects. An objective would be to produce the semiconducting material with a desired geometry (e.g., thickness, width, and/or length) in a first high-throughput step, followed by a second step where the grain structure, defect concentration, etc. are modified. Examples of methods that decouple the processes are described in U.S. application Ser. No. 12/156,499, filed Jun. 2, 2008, titled “Methods of Treating Semiconducting Materials and Treated Semiconducting Materials,” the disclosure of which is incorporated herein by reference.
Additional methods, such as the Zone Melt Recrystallization (ZMR) process and the related approaches described in WO 2009/002550 A1 and “The ZMR 100 Zone melt Recrystallization System for Silicon Films,” Fraunhofer Institut Solar Energiesysteme, melt and resolidify semiconducting material in a lateral direction by scanning a heat source along the substrate plane or vice versa. These ZMR methods suffer from disadvantages, however, such as a less-than-desired throughput.
As described herein, the inventors have now discovered additional methods by which articles of semiconducting materials may be made, and/or methods for treating articles of semiconducting materials. The disclosed methods may facilitate formation of articles of semiconducting materials having desirable attributes such as, for example, one or more of improved crystal grain structure, reduced defects, low surface roughness, and uniform thickness, while reducing material waste and increasing the rate of production.